World production of wood and wood products has increasingly relied upon international trade and the demand for imported wood and wood products can only be expected to increase throughout the world. Moreover, potential supplies for additional wood and wood products are located in remote places such as Russia, New Zealand, Chile and Brazil. An obstacle to importation of green wood and wood products from such varied locations is the danger that foreign products could introduce and spread exotic plant pests throughout the jurisdiction where the products are imported.
Likewise, some jurisdictions like the United States have become large exporters of wood and wood products. However, there are also a number of plant pathogens native to the United States, including the pine wilt nematode (Bursaphelenchus xylophilis) occasionally found in the several species of pine and which are not found in overseas forests. The incursion of this pathogen has caused devastation of natural conifer forests in Portugal and have been reported in certain Scandinavian countries. These pathogens create an obstacle to exporting North American wood fiber to overseas markets in a broader context than the several species of pine, whereas the industry practice of aggregating species and cargos. It is therefore necessary to prophylactically heat treat all woodfiber passing into world trade.
There are several well understood methods for destroying plant pathogens. Unfortunately, each suffer from drawbacks, especially when considered in the context of treating wood and wood products to be transported overseas. For example, it is well settled that heat-treating wood and wood products, typically in a kiln with an attendant reduction in the moisture content of the material, is an effective method for killing plant pathogens. Such heat-treatment processes require bringing the core of the material to a certain minimum temperature for a certain minimum period of time so as to dry the material without causing any cellular or structural degradation.
Even when wood and wood products are heat treated by kiln or dryer, there remains a risk of reinfestation by plant pathogens. Unless the wood and wood products' environment is carefully monitored and controlled, reinfestation can occur before the materials are loaded aboard a cargo vessel for transportation. Even after they are loaded, cross contamination may occur if the vessel contains infested cargo that has not been treated.
One approach to the problem is fumigating wood and wood products once a ship carrying a load of wood or wood products in transit or has completed its journey. This is a customary practice of eliminating pathogens in both the United States and other countries, such as Japan. Fumigation effectively controls plant pathogens that may be associated with the surface and subsurface of debarked logs and other wood products. Fumigation may not be effective in killing other plant pathogens that bore deep into the wood or in killing microscopic organisms that live in the wood's cells. Disadvantages of fumigation include the expense of the fumigant. Another significant drawback of fumigation is that it has been known to pose a health risk to people and the environment. Typical fumigants often include methyl bromide fumigation which can only be carried out under carefully controlled circumstances, usually once the ship has entered port and the crew has been safely removed. This further adds to the environmental risk, time and expense involved in importing wood and wood products which are treated in this manner.
Another obstacle to importing green wood and wood products, especially logs, lumber, wood chips or wood strands from across the seas is the condition of the wood or wood product when it arrives at its destination. A freshly cut log has moisture content of about 50%. As a general rule, because of the evaporation of surface and internal moisture, the longer the period of time since the tree has been cut, the drier the wood becomes. The increasing dryness of a log is a drawback in such subsequent manufacturing processes as the manufacture of lumber or veneer. If care is not taken during the period after felling the tree and continuing up through its shipping, moisture variations cause degrees of wood degradation such as cracks and checks. Moreover, incipient rot can form. These phenomena all make portions of the wood unusable in or less valuable for subsequent fabrication, such as fabrication into lumber or veneer or oriented strand board. Similarly, if care is not taken with wood chips or wood strands, there can be a significant loss of fiber, which can destroy or greatly reduce the value of the wood.
European Union Standing Committee on Plant Health, Council Directive 2000/29/EC of 8 May 2000 is one government mandate related to protective measures against the introduction of organisms harmful to plants or plant products and against their spread within Europe. In terms of wood products, this mandate may be met by land based heat treating systems. These systems are essentially wood chip drying plants, long used in board mills, and pellet mills for preparing the wood fiber for further processing.
Standard drying systems are not suitable for the export requirements of wood fiber and represent impediments in several respects. The capital requirements of an engineer, procure and construct (EPC) program for a heat treating plant have deterred private sector industry from constructing a wood chip heat treating plant proximate to a shipping port. Conventional wood chip drying plants would be of such a large scale to meet the compressed time restrictions under USDA phytosanitary requirements to be logistically inefficient. The present drying plant technology customarily reduces the moisture content of woodchips to below 20% which is an inefficient use of the energy medium and is not necessary to phytosanitary requirements.
Drying mechanisms and structures as current technology applies to shipping, dries the wood fiber in batches or continuous process and concentrates the treated wood chips on a chip pad at a shipping port in advance of loading. The volume constraints of the drying systems require long lead times from drying the fiber and readying the cargo for shipment. As previously mentioned, these systems expose the wood fiber to the risk of being re-infested with prohibited microbes from wood processing operations in communication with the treated cargo.
Accordingly, there has existed a definite need for safe, effective and inexpensive apparatus for eliminating significant plant pathogens risks from green wood and wood products, including large volumes of green logs, sawn lumber, wood chips and wood strands transported overseas. There has also existed a need for a method which minimizes the risk of reinfestation of plant pathogens after the initial treatment. There has existed a still further need for a method for maintaining the fresh-cut characteristics of wood and wood products delivered from overseas by reducing the incidence of cracks, checks and incipient rot or, in the case of wood chips or wood strands, by minimizing fiber loss. It would be advantageous to obviate or mitigate these disadvantages.